How to differentiate between two files in shell script

Question

I have a file a.txt which has

a,20
b,30
c,40

Another b.txt has

a,40
b,60
d,3

I need to iterate the first column and check if a variable is not present in the second file (here c is not present) and send out a mail with that variable name.

Answer 1

To extract just the first column, you can use cut :

cut -d, -f1 file.txt

• -d specifies the delimiter
• -f specifies columns to extract

To check for lines common to a pair of files, use comm :

comm file1 file2

comm outputs three columns: the 1st one lists things unique to file1, the 2nd one things unique to file2, the 3rd things common to both. You can tell comm to omit a column by specifying its number as an argument. comm needs the input files to be sorted, though. I usually set $LC_ALL to C when sorting to avoid any influence of the locale.

So, the answer to your question is

comm -23 <(cut -f1 -d, a.txt | LC_ALL=C sort) <(cut -f1 -d, b.txt | LC_ALL=C sort)

Answer 2

可能的awk解决方案：

awk -F, 'FNR==NR{a[$1];next}!($1 in a){print $1}' b.txt a.txt